Wash and dry all produce. Dice potatoes into 1/2-inch pieces. (TIP: For creamier spuds, peel potatoes first.) Place in a medium pot with enough salted water to cover by 2 inches. Boil until tender, about 15 minutes. Reserve 1/4 cup potato cooking water (1/3 cup for 4 servings), then drain and return potatoes to pot.

Meanwhile, removed and discard any large ribs from kale; chop leaves into 1-inch pieces, if necessary. Halve, peel, and mince shallot. Thinly slice chives. Heat a drizzle of oil in a large pan over medium heat. Add kale and a splash of water. Cook until kale has wilted and is very tender, 4-5 minutes. Season with salt and pepper. Turn off heat, keeping kale in pan.

Pat beef dry with paper towels; season all over with salt and pepper. Heat a drizzle of oil in a medium pan over medium-high heat. Add beef and cook to desired doneness, 4-7 minutes per side. Turn off heat; remove from pan and set aside to rest.

Heat another drizzle of oil in same pan over medium-high heat. Add shallot and cook, stirring, until softened and fragrant, 1-2 minutes. Stir in demi-glace and 1/4 cup water (1/3 cup for 4 servings), scraping up any browned bits from bottom of pan. Bring to a simmer, then reduce heat to medium low. Cook until reduced by half, 2-3 minutes. Turn off heat; stir in half the garlic herb butter.

To pot with potatoes, add 2 TBSP sour cream and 1 TBSP plain butter (2 TBSP plain butter for 4 servings). Mash over medium-low heat, adding reserved potato cooking water as necessary until smooth and creamy. Stir in half the chives; season generously with salt and pepper.

Return pan with kale to medium heat; stir until warm. Stir in remaining sour cream and remaining garlic herb butter. Season with salt and pepper. Divide kale, potatoes, and beef between plates. Drizzle beef with sauce. Garnish with remaining chives.